This invention relates to a self-plugging blind rivet and more particularly a blind rivet of the kind comprising an elongate, generally cylindrical, stem having a radially enlarged head at one of its ends, and a tubular body having a shank, and a radially enlarged pre-formed head at one end of the shank for engaging one face of a workpiece, the stem extending through an axial bore in the body and projecting from the head end of the body, and the head of the stem abutting the shank at the other, or tail, end remote from the pre-formed head, whereby, on pulling the projecting stem and supporting the head of the body in order to set the rivet, a portion of the shank adjacent to the tail-end may be deformed to form a blind head for engaging an opposite, or "blind", face of the workpiece.
In order that a rivet, which is made of a length sufficient to accommodate work of a given maximum thickness, will also perform satisfactorily in workpieces having thicknesses in a range less than the maximum thickness for which the rivet is designed, a portion of the shank intermediate between the tail portion and the pre-formed head is shaped and dimensioned so that, under axial compression, it will collapse axially into an annular bulge or fold which can be forced into engagement with the blind face of a workpiece having a thickness less than the maximum for which the rivet is designed, and thus the workpieces can be fastened between the pre-formed head and the annular bulge, rather than the blind head.
One particularly successful and well-known type of rivet of the foregoing kind is sold under the Registered Trade Mark "AVEX", and has a shank which is initially of cylindrical external shape, and, during manufacture, a portion of the shank intermediate between the pre-formed head and the tail-end is crimped externally, so as to form a waist portion of reduced external diameter as compared with portions adjacent the ends of the shank, which promotes or facilitates the formation of the bulge at an appropriate position, and with an appropriate shape, during setting of the fastener.
The crimping of the shank is necessarily an individual operation in the manufacture of the rivet, and is performed after the fastener has been assembled by insertion of the stem into the bore of the body. Thus, the shank is crimped on to the stem, ensuring that the body will retain the stem and prevent their accidental separation, and during the crimping operation the stem supports the shank, and thus prevents any appreciable reduction in the internal diameter of the shank in the crimped portion, so that the internal diameter of the body remains substantially constant throughout the length of the bore.
The need to perform the crimping operation as an individual step in the manufacture of the rivet is onerous, and we have sought to avoid the need for such a step.
However, the crimping operation also has the effect of causing some work-hardening of the material in the crimped region of the shank. We have found that merely to form a body with a shape which imitates the shape of the crimped body, without actually making the waist portion harder than the rest of the shank, is not a satisfactory alternative to the established method of manufacture, because the behaviour of such a body during setting of the rivet is different from that of the conventionally produced rivet.
We have now found that, in order for a body having a waist portion which is not harder than the rest of the shank to work in a similar way to a body which has been crimped, it is necessary to maintain certain dimensional relationships between the parts of the body, and that these relationships are generally different from those of a crimped body of similar shape.